Treatment of a heater tube intended for a pressurizer of the primary cooling system of a nuclear reactor

Abstract

A treatment of a heater tube intended to be used in a pressurizer of the primary cooling system of a nuclear reactor. In particular, the heater tube comprises a heater housed in a substantially cylindrical sheath. The material of which this sheath is made is a work-hardened austenitic stainless steel. In particular, the external surface of the sheath is liable to undergo a stress corrosion during use of the heatertube. The method includes a heat treatment step, preferably using induction heating, in which the external surface of the sheath is heat-treated so as to recrystallize the material of the sheath at least on the surface thereof.

The invention claimed is: 
     
       1. A method for the treatment of a heating rod intended to be used in a pressurizer of the primary cooling system of a nuclear reactor, said heating rod comprising:
 a substantially cylindrical sheath having an external surface that is liable to undergo stress corrosion, at least partially, during the use of the heating rod, said sheath including a work-hardened austenitic stainless steel material, 
 a mandrel housed inside said sheath, and 
 a heating device housed inside said sheath, said heating device comprising a heating wire coiled around said mandrel and in contact with said sheath, 
 wherein the method comprises a heat treatment step of at least the external surface of said sheath of said heating rod, said heat treatment step including a rise in temperature of the external surface of the sheath to a temperature in a range from 900° C. to 1050° C. to recrystallize the material of the sheath, the heat treatment being implemented using a solenoid inductor coil external to the heating rod and surrounding the substantially cylindrical sheath, and wherein the rise in temperature of the heater resulting from the heat treatment is limited to a maximum value of 850° C. 
 
     
     
       2. The method according to  claim 1 , wherein an inductor coil having a diameter is used and an alternating current frequency applied in the inductor coil for recrystallization treatment using induction heating is at least 100 kHz, for a coil 30 to 50 mm in diameter surrounding the external surface of the sheath, the diameter of the sheath being of the order of 20 to 25 mm. 
     
     
       3. The method according to  claim 1 , wherein an inductor is arranged around the substantially cylindrical sheath, and a relative displacement of the inductor with respect to the substantially cylindrical sheath is applied, at least in translation along the substantially cylindrical sheath. 
     
     
       4. The method according to  claim 3 , wherein a speed of the relative displacement in translation along the substantially cylindrical sheath is between 100 and 900 mm per minute, for a power supplied by induction between 1 and 50 kW. 
     
     
       5. The method according to  claim 1 , further comprising a supply of inert gas onto the external surface of the sheath in order to avoid oxidation following the heat treatment. 
     
     
       6. The method according to  claim 1 , further comprising, after the heat treatment, a step of cooling by blowing fluid onto the external surface of the sheath. 
     
     
       7. The method according to  claim 1 , wherein the external surface of the sheath has at least traces of work-hardening before the heat treatment step. 
     
     
       8. A heating rod produced by the method according to  claim 1 , wherein the sheath of the tube includes at least on its external surface a thickness of recrystallized material. 
     
     
       9. The heating rod according to  claim 8 , wherein the thickness of the sheath is greater than or of the order of 1 mm. 
     
     
       10. The heating rod according to  claim 8 , wherein the sheath has a hardness equivalent to a value less than or equal to approximately 240 Vickers.